For certain applications, and specifically in the field of avionics, there is a need for power transfer devices that offer the following qualities:                a high and electrically insulated output voltage;        a high input dynamic range, preferably greater than two (ratio between maximum input voltage and minimum input voltage);        a transformation ratio between the value of the output voltage and the high input voltage;        an output voltage regulated to a constant value; particularly with little susceptibility to variations of the input voltage source, which can be a battery with a voltage that is very variable in the case of a load with high current draw;        a very small footprint;        a high efficiency to minimize the losses and associated heating effects.        
Such a device is notably sought for the starter systems of the auxiliary power units (APU) in airplanes.
The power transfer devices of the state of the art do not perfectly offer all these qualities. Notably, the French patent application published under the number 2 786 339 discloses a power transfer device of the type with magnetic coupler, with a full-wave rectifier bridge on the secondary side which forms an output voltage regulation circuit. To obtain a converter from a DC source, the device must in practice be associated with two pairs of switches in series, forming an H-configuration bridge. The AC voltage source created by the H-configuration bridge is directly connected to the terminals of the primary winding of the magnetic coupler. The magnetic coupler makes it possible to use the leakage inductance of the transformer as all or part of the storage inductance. The efficiency of the power transfer device is improved. More specifically, and as illustrated in FIG. 1a, A denotes a terminal of the load and B denotes the other terminal of the load, Cs denotes the output filtering capacitor connected between A and B, Np and Ns denote the number of turns of the primary and secondary windings of the transformer, Lf denotes the leakage inductance of the transformer, returned in series with the secondary winding. The rectifier bridge comprises two diodes 1 and 2, each diode connected between a respective end of the series assembly formed by the secondary winding plus leakage inductance, and the terminal A of the load. It also comprises two switches 3 and 4, each connected between a respective end of the secondary winding and the terminal B of the load. The elements 1 and 3, respectively 2 and 4, are in series between A and B. These elements 1 to 4 form a full-wave rectifier bridge. The two switches 3 and 4 are simultaneously ordered to the on state for a phase of predetermined duration, which makes it possible to store energy in the leakage inductance Lf. More specifically, during this phase, the storage inductance is connected to the primary voltage source and can therefore store energy. The duration of this phase can be adjusted according to the application, that is, according to the current requirements in the output load. The two diodes are naturally blocked during the energy storage phase, so as to prevent any transfer of energy to the load.
Such a magnetic coupler offers a good efficiency, of the order of 95%, but it is limited in input voltage dynamic range. In practice, the ratio of the maximum amplitude to the minimum amplitude of the input voltage must be less than or equal to two, for the device to operate optimally, that is, to retain a high efficiency. Furthermore, when the input voltage supplied by the voltage source of the primary is low, the RMS currents increase, and with them, the spectral content of the absorbed current. This causes additional losses in the semiconductors and the inductive elements and imposes a need for severe filtering of the input current, which is also a source of energy losses. In practice, these devices are at their optimum efficiency-wise and design-wise when the ratio of the input voltage and of the output voltage is equal to the transformation ratio of the transformer Np/Ns.
Also known are switched-mode step-up converters, also called “boosts” to use the standard English terminology, which can be associated with circuitry that provides electrical insulation. These boost converters have a simple topology. FIG. 1b considers an uninsulated switched-mode converter of the state of the art. It comprises a step-up inductance 5 in series with the DC voltage source VE, and, following the inductance, on the one hand a diode 6 connected between the inductance and a terminal A of the load, and on the other hand a controlled switch 7 connected in series between the inductance and the other terminal B of the load. The controlled switch and the diode are thus connected in series between the terminal B and the terminal A of the load, and the mid-point of this series assembly is connected to the inductance. An output filtering capacitance CS is provided in parallel between the terminals B and A. A converter of this type does not offer the various qualities sought in the invention. Besides the electrical insulation that is not present in such a converter, the voltage step-up ratio (output voltage divided by the minimum input voltage) that can be envisaged is limited, so that the components of the converter are not subject to too great a stress which would limit their life. In practice, this step-up ratio is limited between 5 and 10, whereas a ratio between output voltage and input voltage of between 20 and 30 is sought.
FIG. 1c represents an insulated switched-mode converter based on the same principle as the boost topology. It is distinguished from the uninsulated converter illustrated in FIG. 1b by a transformer, two switches 7a and 7b on the primary, and two diodes 6a and 6b on the secondary. The transformer comprises two primary windings Np1 and Np2 and two secondary windings Ns1 and Ns2. The primary windings are wound in opposition and have the same number of turns. Similarly, the secondary windings are wound in opposition and have the same number of turns. In the example illustrated, the switch 7a is in series with the first primary winding and the switch 7b is in series with the second primary winding. The diode 6a is in series with the first secondary winding and the diode 6b is in series with the second secondary winding. The operating principle is to switch the switches 7a and 7b to the closed state to store energy in the inductance LB as in a boost, then to switch the switch 7b to the open state, to apply a positive voltage to the first primary winding Np1. This voltage is the output voltage multiplied by the transformation ratio Np1/Ns1. The current in LB passes through the primary winding Np1 of the transformer and the switch 7a. The current in the storage inductance LB is directly transferred to the winding Ns1 and through the diode 6a while the switch 7b is open. Then, a new storage phase in LB is implemented by closing the switches 7a and 7b. Then, the switch 7a is open, a negative voltage on the second primary winding of the transformer is applied and the current in the inductance LB is transferred through the diode 6b. With such a converter, the voltage VS obtained at the output is insulated from the primary voltage. A converter of this type makes it possible, with the transformer, to produce a high voltage ratio, but it is limited in input voltage dynamic range (ratio between the minimum input voltage and the maximum input voltage) by the spurious elements that are intrinsic to the structure. To insulate the output voltage, the transformer has a leakage inductance which modifies the shape of the currents and which generates stress on the switches and the diodes. The leakage inductance provokes a voltage-mode stress on opening of the switches which can be compensated by the addition of elements to help with the switching but which are themselves loss generators.